1. Field of the Invention
The present invention relates, in general, to cooling tubes and is particularly, but not exclusively, applicable to cooling tubes used in a plastic injection-molding machine to cool plastic parts, such as plastic parisons or preforms. More particularly, the present invention relates to a structural configuration of these cooling tubes, and also to method of manufacturing and using such tubes, for example in the context of a manufacturing process for preforms made from polyethylenetetraphthlate (PET) or the like.
2. Related Art
In order to accelerate cycle time, molding machines have evolved to include post mold cooling systems that operate simultaneously with the injection molding cycle. More specifically, while one injection cycle is taking place, the post mold cooling system, typically acting in a complementary fashion with a robotic part removal device, is operative on an earlier formed set of molded articles that have been removed from the mold at a point where they are still relatively hot, but sufficiently solid to allow limited handling.
Post mold temperature conditioning (or cooling) molds, nests or tubes are well known in the art. Typically, such devices are made from aluminum or other materials having good thermal conductivity properties.
To improve cooling efficiency and cycle time performance, EP patent 0 283 644 describes a multi-position take-out plate that has a capacity to store multiple sets of preforms for more than one injection cycle. In other words, each set of preforms is subjected to an increased period of accentuated conduction cooling by retaining the preforms in the cooling tubes for more than one injection cycle. With increased cooling, the quality of the preforms is enhanced. At an appropriate point in time, a set of preforms is ejected (usually by a mechanical ejection mechanism) from the take-out plate onto a conveyor to allow a new set of preforms to be inserted into the now vacant set of cooling tubes. EP patent 0 283 644 is incorporated herein by reference.
In many other cooling tube arrangements, the preform (at some point, if not from the point of introduction) looses contact with the internal side walls of the cooling tube, which loss of thermal contact lessens cooling efficiency and causes uneven cooling. As will be understood, uneven cooling can induce part defects, including deformation of overall shape and crystallization of the plastic (resulting in areas that are visibly hazed). Furthermore, lack of contact can cause ovality across the circumference of the preform, while the loss of the cooling effect can mean that a preform is removed from the cooling tube at an excessively high temperature. In addition to causing surface scratching and overall dimensional deformation, premature removal of a preform at an overly high temperature can also result in the semi-molten exterior of preform sticking either to the tube or another preform; all these effects are clearly undesirable and result in part rejection and increased costs to the manufacturer.
European patent EP 0 266 804 describes an intimate fit cooling tube that is held within an end-of-arm-tool (EOAT) of a robot. The intimate fit cooling tube is water cooled and is arranged to receive a preform shortly after it has attained the glass-transition temperature that allows handling of its form without catastrophic deformation. More particularly, after the preform has undergone some cooling within the closed mold, the mold is opened, the EOAT extended between the cavity and core sides of the mold and the preform off-loaded from a core into the cooling tube that then acts to cool the exterior of the preform by a conduction process. However, as the preform cools it will shrink and therefore may loose contact across its entire circumference with the cooling tube yielding an uneven cooling effect.
U.S. Pat. Nos. 4,102,626 and 4,729,732 are further typical of prior art systems in that they show a cooling tube formed with an external cooling channel machined in the outer surface of the tube body, a sleeve is then assembled to the body to enclose the channel and provide an enclosed sealed path for the liquid coolant to circulate around the body.
WO 97/39874 discloses a tempering mold that has circular cooling channels included within its body. EP 0 700 770 discloses another configuration that includes an inner and outer tube assembly to form cooling channels therebetween.
U.S. Pat. No. 4,208,177 discloses an injection mold cavity containing a porous element that communicates with a cooling fluid passageway subjecting the cooling fluid to different pressures to vary the flow of fluid through the porous plug.
U.S. Pat. No. 4,047,873 discloses an injection blow mold in which the cavity has a sintered porous sidewall that permits a vacuum to draw the parison into contact with the cooling tube sidewall.
U.S. Pat. No. 4,295,811 and U.S. Pat. No. 4,304,542 disclose an injection blow core having a porous metal wall portion.
A “Plastics Technology Online” article entitled “Porous Molds' Big Draw”, by Mikell Knights, printed from the Internet on Jul. 27, 2002, discloses a porous tooling composite called METAPOR™. The article discloses the technique of polishing this material to close slightly the pores to improve the surface finish and reduce the porosity.
An article from International Mold Steel, Inc., entitled “Porous Aluminum Mold Materials”, by Scott W. Hopkins, printed from the Internet on Jul. 27, 2002, also discloses porous aluminum mold materials. The materials and applications disclosed in the above two articles refer to vacuum thermoforming of plastics in the mold itself, in which preheated sheets of plastic are drawn into a single mold half via a vacuum drawn through the porous structure of the mold half.